Benzo[a]pyrene/aryl hydrocarbon receptor signaling inhibits osteoblastic differentiation and collagen synthesis of human periodontal ligament cells

The Role of Aryl Hydrocarbon Receptor in Bone Biology

Aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, is crucial in maintaining the skeletal system. Our study focuses on encapsulating the role of AhR in bone biology and identifying novel signaling pathways in musculoskeletal pathologies using the GEO dataset. The GEO2R analysis identified 8 genes (CYP1C1, SULT6B1, CYB5A, EDN1, CXCR4B, CTGFA, TIPARP, and CXXC5A) involved in the AhR pathway, which play a pivotal role in bone remodeling. The AhR knockout in hematopoietic stem cells showed alteration in several novel bone-related transcriptomes (eg, Defb14, ZNF 51, and Chrm5). Gene Ontology Enrichment Analysis demonstrated 54 different biological processes associated with bone homeostasis. Mainly, these processes include bone morphogenesis, bone development, bone trabeculae formation, bone resorption, bone maturation, bone mineralization, and bone marrow development. Employing Functional Annotation and Clustering through DAVID, we further uncovered the involvement of the xenobiotic metabolic process, p450 pathway, oxidation-reduction, and nitric oxide biosynthesis process in the AhR signaling pathway. The conflicting evidence of current research of AhR signaling on bone (positive and negative effects) homeostasis may be due to variations in ligand binding affinity, binding sites, half-life, chemical structure, and other unknown factors. In summary, our study provides a comprehensive understanding of the underlying mechanisms of the AhR pathway in bone biology.

The Potential Influence of Uremic Toxins on the Homeostasis of Bones and Muscles in Chronic Kidney Disease

Patients with chronic kidney disease (CKD) often experience a high accumulation of protein-bound uremic toxins (PBUTs), specifically indoxyl sulfate (IS) and p-cresyl sulfate (pCS). In the early stages of CKD, the buildup of PBUTs inhibits bone and muscle function. As CKD progresses, elevated PBUT levels further hinder bone turnover and exacerbate muscle wasting. In the late stage of CKD, hyperparathyroidism worsens PBUT-induced muscle damage but can improve low bone turnover. PBUTs play a significant role in reducing both the quantity and quality of bone by affecting osteoblast and osteoclast lineage. IS, in particular, interferes with osteoblastogenesis by activating aryl hydrocarbon receptor (AhR) signaling, which reduces the expression of Runx2 and impedes osteoblast differentiation. High PBUT levels can also reduce calcitriol production, increase the expression of Wnt antagonists (SOST, DKK1), and decrease klotho expression, all of which contribute to low bone turnover disorders. Furthermore, PBUT accumulation leads to continuous muscle protein breakdown through the excessive production of reactive oxygen species (ROS) and inflammatory cytokines. Interactions between muscles and bones, mediated by various factors released from individual tissues, play a crucial role in the mutual modulation of bone and muscle in CKD. Exercise and nutritional therapy have the potential to yield favorable outcomes. Understanding the underlying mechanisms of bone and muscle loss in CKD can aid in developing new therapies for musculoskeletal diseases, particularly those related to bone loss and muscle wasting.

Osteoporosis and Alveolar Bone Health in Periodontitis Niche: A Predisposing Factors-Centered Review

Periodontitis is a periodontal inflammatory condition that results from disrupted periodontal host-microbe homeostasis, manifested by the destruction of tooth-supporting structures, especially inflammatory alveolar bone loss. Osteoporosis is characterized by systemic deterioration of bone mass and microarchitecture. The roles of many systemic factors have been identified in the pathogenesis of osteoporosis, including endocrine change, metabolic disorders, health-impaired behaviors and mental stress. The prevalence rate of osteoporotic fracture is in sustained elevation in the past decades. Recent studies suggest that individuals with concomitant osteoporosis are more vulnerable to periodontal impairment. Current reviews of worse periodontal status in the context of osteoporosis are limited, mainly centering on the impacts of menopausal and diabetic osteoporosis on periodontitis. Herein, this review article makes an effort to provide a comprehensive view of the relationship between osteoporosis and periodontitis, with a focus on clarifying how those risk factors in osteoporotic populations modify the alveolar bone homeostasis in the periodontitis niche.

Benzo[a]pyrene exposure promotes RIP1-mediated necroptotic death of osteocytes and the JNK/IL-18 pathway activation via generation of reactive oxygen species

Benzo[a]pyrene (BaP) is a polycyclic aromatic hydrocarbon (PAH) of environmental pollutants, readily produced during the processing of petroleum and fatty foods. BaP exposure can cause skeletal deformities. However, whether BaP affects osteocytes, making up over 95% of all the bone cells, remains unknown. This study aimed to investigate the effect of BaP on osteocytes in vivo and in vitro, as well as explore the underlying mechanisms. The in vivo data showed that BaP (50mg/kg) exposure for 12 weeks could cause bone destruction, and increase osteocytes death in mouse cortical femur. Our in vitro results revealed that BaP (25-100 μmol/L) exposure inhibited cell viability of MLO-Y4 cells, and resulted in cell death in a dose-dependent manner. Furthermore, BaP exposure significantly triggered necroptosis of MLO-Y4 cells, as indicated by increased propidium iodide (PI)-positive cells and up-regulation of necroptosis-related protein expressions of receptor-interacting protein kinase 1 (RIP1), RIP3, and mixed lineage kinase domain-like protein (MLKL). This necrotic effect was reversed by the RIP1 inhibitor necrostatin-1 (Nec-1). Simultaneously, BaP activated the downstream c-Jun N-terminal kinase (JNK)/ interleukin (IL)-18 signaling pathway, which was suppressed after the JNK inhibitor SP600125 or Nec-1 treatment. In addition, BaP exposure promoted the production of intracellular reactive oxygen species (ROS), mitochondrial ROS (mtROS), and elevated malondialdehyde (MDA) levels; while BaP decreased superoxide dismutase (SOD) activity and antioxidant enzymes including nuclear factor E₂-related factor 2 (Nrf2) and heme oxygenase-1 (HO-1) levels, leading to oxidative damage. The ROS scavenger N-acetylcysteine (NAC) inhibited this necroptotic death and the JNK/IL-18 pathway activation. Collectively, BaP exposure may cause RIP1-mediated necroptotic death of osteocytes and activate the JNK/IL-18 pathway via ROS generation.

New insights into benzo[⍺]pyrene osteotoxicity in zebrafish

Persistent and ubiquitous organic pollutants, such as the polycyclic aromatic hydrocarbon benzo[⍺]pyrene (BaP), represent a major threat to aquatic organisms and human health. Beside some well-documented adverse effects on the development and reproduction of aquatic organisms, BaP was recently shown to affect fish bone formation and skeletal development through mechanisms that remain poorly understood. In this work, zebrafish bone-related in vivo assays were used to evaluate the osteotoxic effects of BaP during bone development and regeneration. Acute exposure of zebrafish larvae to BaP from 3 to 6 days post-fertilization (dpf) induced a dose-dependent reduction of the opercular bone size and a depletion of osteocalcin-positive cells, indicating an effect on osteoblast maturation. Chronic exposure of zebrafish larvae to BaP from 3 to 30 dpf affected the development of the axial skeleton and increased the incidence and severity of skeletal deformities. In young adults, BaP affected the mineralization of newly formed fin rays and scales, and impaired fin ray patterning and scale shape, through mechanisms that involve an imbalanced bone remodeling. Gene expression analyses indicated that BaP induced the activation of xenobiotic and metabolic pathways, while negatively impacting extracellular matrix formation and organization. Interestingly, BaP exposure positively regulated inflammation markers in larvae and increased the recruitment of neutrophils. A direct interaction between neutrophils and bone extracellular matrix or bone forming cells was observed in vivo, suggesting a role for neutrophils in the mechanisms underlying BaP osteotoxicity. Our work provides novel data on the cellular and molecular players involved in BaP osteotoxicity and brings new insights into a possible role for neutrophils in inflammatory bone reduction.

Toxic Effects of Indoxyl Sulfate on Osteoclastogenesis and Osteoblastogenesis

Uremic toxins, such as indoxyl sulfate (IS) and kynurenine, accumulate in the blood in the event of kidney failure and contribute to further bone damage. To maintain the homeostasis of the skeletal system, bone remodeling is a persistent process of bone formation and bone resorption that depends on a dynamic balance of osteoblasts and osteoclasts. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that regulates the toxic effects of uremic toxins. IS is an endogenous AhR ligand and is metabolized from tryptophan. In osteoclastogenesis, IS affects the expression of the osteoclast precursor nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) through AhR signaling. It is possible to increase osteoclast differentiation with short-term and low-dose IS exposure and to decrease differentiation with long-term and/or high-dose IS exposure. Coincidentally, during osteoblastogenesis, through the AhR signaling pathway, IS inhibits the phosphorylation of ERK, and p38 reduces the expression of the transcription factor 2 (Runx2), disturbing osteoblastogenesis. The AhR antagonist resveratrol has a protective effect on the IS/AhR pathway. Therefore, it is necessary to understand the multifaceted role of AhR in CKD, as knowledge of these transcription signals could provide a safe and effective method to prevent and treat CKD mineral bone disease.

Benzo[a]pyrene induces fibrotic changes and impairs differentiation in lung stem cells

Human activities have generated air pollution, with extremely small particles (PM 2.5, particulate matter less than 2.5 µm in diameter) and liquid droplets, which become a menace to human health. Among the pollutants, polycyclic aromatic hydrocarbons (PAHs), which enhance the risks of pulmonary dysfunction and cancer development, have been extensively studied. Numerous studies have addressed the effects of PAHs on the respiratory system, whereas the effects on lung stem/progenitor cells remain unknown. Here, we provide evidence that benzo[a]pyrene (BaP), a major toxic PAH, induces fibrotic changes with a loss of α-1,6-fucosylation in CD54⁺CD157⁺CD45^- cells (lung stem cells). In studies with aryl hydrocarbon receptor (AHR) antagonist, we found that these effects by BaP are independent of the canonical AHR pathway. In addition, these BaP-induced fibrotic changes are reduced by TGF-β antagonist, suggesting an alternative pathway of BaP toxicity is different from other PAH/AHR signaling pathways. Finally, it was observed that BaP impairs the spheroid formation and the podoplanin expression of CD54⁺CD157⁺CD45^- cells, indicating that BaP suppresses the differentiation of lung stem cells. Taken together, our findings reveal specific BaP-induced injuries in CD54⁺CD157⁺CD45^- cells.

Bone Disruption and Environmental Pollutants

BACKGROUND

Endocrine Disrupting Chemicals (EDCs) are ubiquitous and may significantly contribute in environmental pollution, thus contaminating humans and wildlife. Environmental pollutants could interfere with bone homeostasis by means of different mechanisms, which include hormonal imbalance, direct osteoblasts toxicity and enanchment of osteoclasts activity, thus leading to osteopenia or osteoporosis. Among these, bisphenols, dioxins, polycyclic aromatic hydrocarbons, polychlorobiphenyls, poly- and perfluoroalkyls, phthalates, parabens, organotins and cadmium may play a role in bone distuption.

METHODS

PubMed/MEDLINE, ISI-web of knowledge and Google scholar databases were searched for medical subject headings terms and free-text word related to the aforementioned classes of chemicals and bone metabolism and remodelling for better clarifying and understanding the main mechanisms of bone disruption.

RESULTS

Several of EDCs act as xenoestrogens. Considering that estrogens play a significant role in regulating bone remodeling, most of these chemicals generate hormonal imbalance with possible detrimental consequences on bone tissue structure and its mechanical and non-mechanical properties.

DISCUSSION

A lot of evidences about bone distruptors came from in vitro studies or animal models, and conduct to equivocal results. In addition, a few data derived form humans and most of these data focused on the impact of EDCs on bone mineral density without considering their influence on long-term fracture risk. Moreover, it should be taken into account that humans are exposed to a mixture of EDCs and the final effect on bone metabolism might be the result of either a synergism or antagonist effects among them. Age of first exposure, cumulative dose exposure over time, and the usually observed non-monotonic dose-response curve for EDCs should be considered as other important variable influencing the final effect on bone metabolism.

CONCLUSION

Taking into account these variables, observational studies are needed to better analyze this issue both for echological purpose and to preserve bone health.

Benzo[a]pyrene injures BMP2-induced osteogenic differentiation of mesenchymal stem cells through AhR reducing BMPRII

Benzo[a]pyrene(BaP), a polycyclic aromatic hydrocarbons (PAH) of environmental pollutants, is one of the main ingredients in cigarettes and an agonist of the aryl hydrocarbon receptor (AhR). Mesenchymal stem cells (MSCs) including C3H10T1/2 and MEF cells, adult multipotent stem cells, can be differentiated toward osteoblasts during the induction of osteogenic induction factor-bone morphogenetic protein 2(BMP2). Accumulating evidence suggests that BaP decreases bone development in mammals, but the further mechanisms of BaP on BMP2-induced bone formation involved are unknown. Here, we researched the role of BaP on BMP2-induced osteoblast differentiation and bone formation. We showed that BaP significantly suppressed early and late osteogenic differentiation, and downregulated the runt-related transcription factor 2(Runx2), osteocalcin(OCN) and osteopontin (OPN) during the induction of BMP2 in MSCs. Consistent with in vitro results, administration of BaP inhibited BMP2-induced subcutaneous ectopic osteogenesis in vivo. Interestingly, blocking AhR reversed the inhibition of BaP on BMP2-induced osteogenic differentiation, which suggested that AhR played an important role in this process. Moreover, BaP significantly decreased BMP2-induced Smad1/5/8 phosphorylation. Furthermore, BaP significantly reduced bone morphogenetic protein receptor 2(BMPRII) expression and excessively activated Hey1. Thus, our data demonstrate the role of BaP in BMP2-induced bone formation and suggest that impaired BMP/Smad pathways through AhR regulating BMPRII and Hey1 may be an underlying mechanism for BaP inhibiting BMP2-induced osteogenic differentiation.

Periodontitis is an inflammatory disease of oxidative stress: We should treat it that way

Periodontitis is a highly prevalent disease. As it progresses, it causes serious morbidity in the form of periodontal abscesses and tooth loss and, in the latter stages, pain. It is also now known that periodontitis is strongly associated with several nonoral diseases. Thus, patients with periodontitis are at greater risk for the development and/or exacerbation of diabetes, chronic obstructive pulmonary disease, and cardiovascular diseases, among other conditions. Although it is without question that specific groups of oral bacteria which populate dental plaque play a causative role in the development of periodontitis, it is now thought that once this disease has been triggered, other factors play an equal, and possibly more important, role in its progression, particularly in severe cases or in cases that prove difficult to treat. In this regard, we allude to the host response, specifically the notion that the host, once infected with oral periodontal pathogenic bacteria, will mount a defense response mediated largely through the innate immune system. The most abundant cell type of the innate immune system - polymorphonuclear neutrophils - can, when protecting the host from microbial invasion, mount a response that includes upregulation of proinflammatory cytokines, matrix metalloproteinases, and reactive oxygen species, all of which then contribute to the tissue damage and loss of teeth commonly associated with periodontitis. Of the mechanisms referred to here, we suggest that upregulation of reactive oxygen species might play one of the most important roles in the establishment and progression of periodontitis (as well as in other diseases of inflammation) through the development of oxidative stress. In this overview, we discuss both innate and epigenetic factors (eg, diabetes, smoking) that lead to the development of oxidative stress. This oxidative stress then provides an environment conducive to the destructive processes observed in periodontitis. Therefore, we shall describe some of the fundamental characteristics of oxidative stress and its effects on the periodontium, discuss the diseases and other factors that cause oxidative stress, and, finally, review potentially novel therapeutic approaches for the management (and possibly even the reversal) of periodontitis, which rely on the use of therapies, such as resveratrol and other antioxidants, that provide increased antioxidant activity in the host.

Protective roles of FICZ and aryl hydrocarbon receptor axis on alveolar bone loss and inflammation in experimental periodontitis

AIM

The aryl hydrocarbon receptor (AhR)-ligand axis has been shown to be involved in inflammatory diseases and bone homeostasis. However, the activation of AhR signalling pathway and the possible functions of AhR ligands in periodontitis are underexplored. This study investigated the expression of the AhR target gene cytochrome P450 subfamily B member 1 (CYP1B1) and the functions and mechanisms of the AhR ligand 6 formylindolo[3,2-b]carbazole (FICZ) in periodontitis.

MATERIALS AND METHODS

CYP1B1 expression was detected in human periodontitis samples, mice with ligature-induced periodontitis and lipopolysaccharide (LPS)-induced inflammation in periodontal ligament cells (PDLCs) in vitro. FICZ was administered topically or systemically. The therapeutic functions of FICZ were detected via qPCR, micro-computed tomography and immunohistochemistry. Finally, the mechanisms of AhR signalling in periodontitis were investigated by cell assays.

RESULTS

CYP1B1 expression was downregulated in periodontitis. FICZ rescued the alveolar bone loss and mitigated the inflammatory cytokines in periodontitis mice. In vitro, FICZ pre-treatment reduced the LPS-induced inflammation in PDLCs via the increased phosphorylation of STAT3. Additionally, FICZ prompted the mineralization of PDLCs via activation of the Wnt/β-catenin signalling pathway.

CONCLUSION

AhR signalling pathway is suppressed in periodontitis and the AhR ligand FICZ can prevent periodontitis.

Induction of maxillary and mandibular squamous epithelial cell proliferation in mink (Neovison vison) by β-naphthoflavone

A jaw lesion reported in mink exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and TCDD-like chemicals is considered a potential indicator of exposure to these chemicals. Many of the effects of TCDD-like chemicals are induced through interaction with the aryl hydrocarbon receptor. The present study indicates that mink dosed with β-naphthoflavone, which is an aryl hydrocarbon receptor ligand but not a TCDD-like chemical, also develop the lesion. Environ Toxicol Chem 2019;38:460-463. © 2018 SETAC.

Influence of nicotine on orthodontic tooth movement: A systematic review of experimental studies in rats

OBJECTIVE

The objective of this systematic review was to assess the impact of nicotine administration on orthodontic tooth movement (OTM).

METHODS

A systematic search was conducted in PubMed, Scopus, EMBASE, MEDLINE (OVID) and Web of Knowledge databases and the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were followed. Studies evaluating the influence of nicotine on OTM, and with the presence of a control group (OTM without nicotine administration), were included. Quality assessment of the selected studies was performed following the Animal Research Reporting in Vivo Experiment (ARRIVE) guidelines.

RESULTS

Six of the initially identified 108 articles fulfilled the inclusion criteria and were selected. All included studies were performed in male rats, which underwent OTM with or without nicotine administration. Since there was a variation among the included studies regarding nicotine dosage and the duration and magnitude of force application during OTM only a qualitative analysis could be performed. The studies reported that nicotine administration accelerated OTM by inducing alveolar bone resorption around the moving teeth. It was also found that nicotine increased root resorption during experimental OTM. More standardized animal research or clinical studies are warranted to further evaluate the impact of nicotine on OTM.

CONCLUSIONS

On an experimental level, nicotine exposure in rats jeopardizes OTM by increasing alveolar bone loss and root resorption. From a clinical perspective, further studies are needed to assess the impact of habitual use of tobacco products on OTM.

Suppression of aberrant choroidal neovascularization through activation of the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR) is a ligand activated transcription factor, initially discovered for its role in regulating xenobiotic metabolism. There is extensive evidence supporting a multi-faceted role for AhR, modulating physiological pathways important in cell health and disease. Recently we demonstrated that the AhR plays a role in the pathogenesis of age-related macular degeneration (AMD), the leading cause of vision loss in the elderly. We found that loss of AhR exacerbates choroidal neovascular (CNV) lesion formation in a murine model. Herein we tested the therapeutic impact of AhR activation on CNV lesion formation and factors associated with aberrant neovascularization. We screened a panel of synthetic drugs and endogenous AhR ligands, assessed their ability to activate AhR in choroidal endothelial cells, and inhibit angiogenesis in vitro. Drugs with an anti-angiogenic profile were then administered to a murine model of CNV. Two compounds, leflunomide and flutamide, significantly inhibited CNV formation concurrent with positive modifying effects on angiogenesis, inflammation, extracellular matrix remodeling, and fibrosis. These results validate the role of the AhR pathway in regulating CNV pathogenesis, identify mechanisms of AhR-based therapies in the eye, and argue in favor of developing AhR as a drug target for the treatment of neovascular AMD.

The role of aryl hydrocarbon receptor in bone remodeling

Bone remodeling is a persistent process for maintaining skeletal system homeostasis, and it depends on the dynamic equilibrium between bone-forming osteoblasts and bone-resorbing osteoclasts. Aryl hydrocarbon receptor (Ahr), a ligand-activated transcription factor, plays a pivotal role in regulating skeletal system. In order to better understand the role of Ahr in bone remodeling, we focused on bone remodeling characteristic, and the effects of Ahr on bone formation and differentiation, which suggest that Ahr is a critical control factor in the process of bone remodeling. Moreover, we discussed the impacts of Ahr on several signaling pathways related to bone remodeling, hoping to provide a theoretical basis to improve bone remodeling.

Comparative proteomic analysis and characterization of benzo(a)pyrene removal by Microbacterium sp. strain M.CSW3 under denitrifying conditions

High-molecular-weight polycyclic aromatic hydrocarbons are persistent organic pollutants with great environmental and human health risks and the associated bioremediation activities have always been hampered by the lack of powerful bacterial species under redox conditions. A Microbacterium sp. strain capable of using benzo(a)pyrene as sole carbon and energy sources under denitrifying conditions was isolated. The difference in protein expression during BaP removal and removal characterization were investigated. A total of 146 proteins were differentially expressed, 44 proteins were significantly up-regulated and 102 proteins were markedly down-regulated. GO and COG analysis showed that BaP removal inhibited the expression of proteins related to glucose metabolism at different levels and activated other metabolic pathway. The proteins associated with catalytic activity and metabolic process were altered significantly. Furthermore, the BaP removal might be occurred in certain organelle of M.CSW3. The strain removed BaP with a speed of 0.0657-1.0072 mg/L/day over the concentrations range 2.5-100 mg/L. High removal rates (>70%) were obtained over the range of pH 7-11 in 14 days. Carbohydrates and organic acids which could be utilized by the strain, as well as heavy metal ions, reduced BaP removal efficiency. However, phenanthrene or pyrene addition enhanced the removal capability of M.CSW3. The strain was proved to have practical potential for bioremediation of PAHs-contaminated soil and this study provided a powerful platform for further application by improving production of associated proteins.

Comparative systems toxicology analysis of cigarette smoke and aerosol from a candidate modified risk tobacco product in organotypic human gingival epithelial cultures: A 3-day repeated exposure study

Smoking is one of the major lifestyle-related risk factors for periodontal diseases. Modified risk tobacco products (MRTP) offer a promising alternative in the harm reduction strategy for adult smokers unable to quit. Using a systems toxicology approach, we investigated and compared the exposure effects of a reference cigarette (3R4F) and a heat-not-burn technology-based candidate MRTP, the Tobacco Heating System (THS) 2.2. Human gingival epithelial organotypic cultures were repeatedly exposed (3 days) for 28 min at two matching concentrations of cigarette smoke (CS) or THS2.2 aerosol. Results showed only minor histopathological alterations and minimal cytotoxicity upon THS2.2 aerosol exposure compared to CS (1% for THS2.2 aerosol vs. 30% for CS, at the high concentration). Among the 14 proinflammatory mediators analyzed, only 5 exhibited significant alterations with THS2.2 exposure compared with 11 upon CS exposure. Transcriptomic and metabolomic analysis indicated a general reduction of the impact in THS2.2 aerosol-exposed samples with respect to CS (∼79% lower biological impact for the high THS2.2 aerosol concentration compared to CS, and 13 metabolites significantly perturbed for THS2.2 vs. 181 for CS). This study indicates that exposure to THS2.2 aerosol had a lower impact on the pathophysiology of human gingival organotypic cultures than CS.